Chipmunk International | Home | Technical Issues
Due to the way the world works one has to figure this out in a number of steps.
Step 1: Figure out the manufacturer of the chips.
Step 2: Figure out the specs of the individual chips. (EDO/FPM, Refresh Cycles and Voltage)
Step 3: Figure out the specs of the SIMM or DIMM module. (Single/Double bank and Size)
Step 4: What kind of memory module do I need for this Mac?
Explanation of the important parameters:
Voltage
Chips and hence the whole module needs a 5 volt supply or a 3.3 volt supply. Remember that using a 3.3 v module in a 5 v computer will result in the immediate death of the module. Using 5 v modules in a 3.3 v computer will at best work unreliable.
EDO / FMP
Extended Data Out (EDO) could theoretically be faster than Fast Page Mode (FPM) but not all computers will make use of this. In fact there are some computers (PowerMac 7200) that do not accept EDO memory and these could even be destroyed by using it.
EDO is a DRAM performance feature that permits multiple bits of data in a single row to be accessed quickly. EDO involves selecting multiple column addresses in rapid succession once the row address has been selected. Once the first column address has been selected and CAS becomes active, the data output drivers are activated. The data output drivers remain active for each successive CAS strobe, until RAS goes high.
FPM is a timing option that permits several bits of data in a single row on a DRAM to be accessed at an accelerated rate. Fast Page Mode involves selecting multiple column addresses in rapid succession once the row address has been selected. Each time a column address is selected and CAS becomes active, the data output drivers are activated; each time CAS goes high, the data output drivers are deactivated.
Refresh modes
Use this page to discover how many refresh cycles are needed for any particular model of chip and module. Remember that you have to refresh at least the amount of columns required by the chips. Using for example a 4K refresh SIMM in a 2K refresh Mac will result is data loss and unexpected crashes. The lower the amount of Refresh Cycles needed by a chip the better. If the computer refreshes 2K then 2K and 1K will be OK, but 4K will give memory errors.
Internal architecture
Use this page to see the internal architecture a chip. This will help you understand how a DIMM or SIMM is built up and how big it is. You have to know the internal architecture before you can go on to discover if a DIMM or SIMM is single or double banked.
Single of Double bank modules
A SIMM or DIMM can contain one or two 'Banks'. Having two banks is like having two SIMM's stuck together on one board. This has nothing to do with the module having chips on both sides of the board.
So why would I want to know this about my modules?
There are many issues with placing memory modules in Macintosh computers that are not clear and that Apple is not disclosing. Strange things are happening and they have everything to do with these different parameters. And if there are strange problems with memory in the Mac, like modules working individually but not together, or modules working in some banks but not in others, you have no hope of understanding what is going on unless you have these parameters.
Known problems
When two modules are supposed to be using interleaving it is necessary for all the parameters for the modules to be identical. Trying to use interleaving for modules that are similar but not identical will give mixed results.
On the Tanzania Motherboard (PowerMac 4400 / StarMax and others) 64MB DIMM's have to be 2K refresh or better.
On the Tanzania Motherboard (PowerMac 4400 / StarMax and others) 1 of the 3 DIMM banks is only allowed to contain a single bank module.
In the Performa/LC/Quadra 630 models that have two SIMM banks (most only have one) the second slot in only allowed to contain a single bank module.
In the PowerMac 7200 no EDO memory is allowed. If you use it you risk damaging the Mac and the memory.
SIMM's (both 30 pin and 72 pin) when used in a Mac must always be FPM. EDO is not allowed there.
SIMMs in a DIMM tree must all have exactly the same specifications. The EDO/FPM, refresh cycles and the voltage for the whole tree will be identical to that of the individual SIMMs. The amount of banks you get will depend on the circuits on the tree. If there are two 72 pin SIMMs in a DIMM tree then the amount of banks will remain the same. If there are 4 SIMMs the amount of banks will be doubled. It could very well be that only single bank SIMMs are allowed in DIMM trees in the first place.
Please tell me if you know or discover problems not mentioned here.
Any questions? - ram@chipmunk.nl
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